Acanthaster planci Outbreak: Decline in Coral Health, Coral Size Structure Modification and Consequences for Obligate Decapod Assemblages

Although benthic motile invertebrate communities encompass the vast majority of coral reef diversity, their response to habitat modification has been poorly studied. A variety of benthic species, particularly decapods, provide benefits to their coral host enabling them to cope with environmental stressors, and as a result benefit the overall diversity of coral-associated species. However, little is known about how invertebrate assemblages associated with corals will be affected by global perturbations, (either directly or indirectly via their coral host) or their consequences for ecosystem resilience. Analysis of a ten year dataset reveals that the greatest perturbation at Moorea over this time was an outbreak of the corallivorous sea star Acanthaster planci from 2006 to 2009 impacting habitat health, availability and size structure of Pocillopora spp. populations and highlights a positive relationship between coral head size and survival. We then present the results of a mensurative study in 2009 conducted at the end of the perturbation (A. planci outbreak) describing how coral-decapod communities change with percent coral mortality for a selected coral species, Pocillopora eydouxi. The loss of coral tissue as a consequence of A. planci consumption led to an increase in rarefied total species diversity, but caused drastic modifications in community composition driven by a shift from coral obligate to non-obligate decapod species. Our study highlights that larger corals left with live tissue in 2009, formed a restricted habitat where coral obligate decapods, including mutualists, could subsist. We conclude that the size structure of Pocillopora populations at the time of an A. planci outbreak may greatly condition the magnitude of coral mortality as well as the persistence of local populations of obligate decapods

An indicator of porosity through simulation of melt pool volume in aluminum wire arc additive manufacturing

International audienceManaging the quality of functional parts is a key challenge in wire arc additive manufacturing. In case of additive production of aluminum parts, porosity is one of the main limitations of this process. This paper provides an indicator of porosity through the simulation of melt pool volume in aluminum wire arc additive manufacturing. First, a review of porosity formation during WAAM process is presented. This review leads to the proposal of this article: monitoring the porosity inside produced part can be achieved through the melt pool volume monitoring. An adapted Finite Element model is then proposed to determine the evolution of the melt pool volume throughout the manufacturing process of the part. This model is validated by experimental temperature measurement. Then, in order to study the link between the porosity and the melt pool volume, two test parts are chosen to access to two different pore distributions. These two parts are simulated and produced. The porosity rates of produced parts are then measured by X-ray tomography and compared to the simulated melt pool volumes. The analysis of the results highlights the interest of the melt pool volume as a predictive indicator of the porosity rate

Caractérisation d'un poste de soudure Cold Métal Transfer pour le pilotage du procédé Wire Arc Additive Manufacturing

International audienceCaractérisation d'un poste de soudure Cold Métal Transfer pour le pilotage du procédé Wire Arc Additive Manufacturin

Qualify a NIR camera to detect thermal deviation during aluminum WAAM

International audienceThis paper proposes to qualify the minimal quality deviation that can be detected by a near-infrared camera during aluminum wire arc additive manufacturing. First, a review of the literature is done to highlight the interest in monitoring the melt pool in industrial condition for thermal management during manufacturing. It points out the relevance of the use of a near-infrared camera for steels, but it has to be demonstrated for aluminum alloys. Indeed, the melt pool of the aluminum is significantly dimmer and less distinct than the melt pool of the steels. An experimental design is proposed to qualify the minimal quality deviation that can be detected on a thin wall. The chosen default to correlate with the thermal deviation is the width of the wall. A method is proposed to extract a thermal metric from the camera image and to analyze its sensitivity to a width deviation of the wall. The paper shows the correlation between the width of the wall and the thermal metric for different heat conditions. Moreover, the thermal metric is sensitive to width deviation either on the wall scale or on the bead scale. It indicates the relevance of a near-infrared camera to detect heat accumulation-induced width deviation during wire arc additive manufacturing of aluminum alloy

Ordination plots representing the composition of decapod communities with the loss of coral tissue.

<p>Non-metric multidimensional scaling plots were computed using Jaccard, an incidence based metric (A) and Bray-Curtis, an abundance based metric (B). Light grey, dark grey and black dots represent non-eaten, partially eaten and dead <i>Pocillopora</i> respectively. Mean ± SE beta diversity (average distance to group centroid) is plotted below the corresponding ordination plot (C and D).</p

Effect of the loss of live coral tissue on overall decapod species richness.

<p>(A) Total decapod species richness: polynomial regression, <i>y</i> = −0.0025<i>x</i>²+0.1214<i>x</i>+22.258; <i>R²</i> = 0.72, N = 82, <i>p</i><0.001. (B) Rarefied total decapod species richness: polynomial regression, <i>y</i> = −0.0012<i>x</i>²+0.0746<i>x</i>+10.352; <i>R²</i> = 0.67, N = 82, <i>p</i><0.001.</p

Effects of a recent outbreak of <i>Acanthaster planci</i> from 2006–2009 on <i>Pocillopora</i> populations.

<p>Size-class (cm²) distributions of live (white), partially dead (grey) and completely dead (black) corals (<i>Pocillopora</i> spp.) as a proportion of the total number of coral heads at each site. High resolution photographs of 60 permanent quadrats were taken at three sites in Moorea (20 quadrats/site) every two years from 2000 to 2008 at the sites Vaipahu and Haapiti and from 2001–2009 at Tiahura. Numbers in parentheses represent the total number of <i>Pocillopora</i> measured per site.</p